In an effort to increase areal density of magnetic storage media, it is desirable to reduce the volume of magnetic material used to store bits of information in magnetic storage media. However, with the available materials, current recording heads are not able to provide a sufficient or high enough magnetic writing field to write on such a medium. Accordingly, it has been proposed to overcome the recording head field limitations by employing thermal energy to heat a local area on the recording medium before, or at about the time of, applying the magnetic write field to the medium. By heating the medium, the coercivity is reduced such that the magnetic write field is sufficient to write to the medium. Once the medium cools to ambient temperature, the medium has a sufficiently high value of coercivity to assure thermal stability of the recorded information.
Heat assisted magnetic recording allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature to assure sufficient thermal stability. Heat assisted magnetic recording can be applied to any type of magnetic storage media, including tilted media, longitudinal media, perpendicular media and patterned media.
For heat assisted magnetic recording, an electromagnetic wave of, for example, visible, infrared or ultraviolet light can be directed onto a surface of a data storage medium to raise the temperature of the localized area of the medium to facilitate switching of the magnetization of the area. Solid immersion lenses (SILs) have been proposed for use in reducing the size of a spot on the medium that is subjected to the electromagnetic radiation. In addition, solid immersion mirrors (SIMs) have been proposed to reduce the spot size. SILs and SIMs may be either three-dimensional or two-dimensional. In the latter case they correspond to mode index lenses or mirrors in planar waveguides. A metal pin can be inserted at the focus of a SIM to guide a confined beam of light out of the SIM to the surface of the recording medium.
A planar solid immersion mirror (PSIM) is a slab waveguide terminated by a parabolic mirror so that an incident slab waveguide mode focuses at the parabolic focus. The input slab waveguide mode is launched by a Gaussian spot incident on the grating coupler, and the amplitude of the slab waveguide mode also has a Gaussian form. Thus, the peak amplitude lies along the symmetry axis of the parabola and the amplitude decays towards the sides. The focal spot for a Gaussian input shows strong side-lobes that are within half a wavelength of the main lobe.
For the design of an integrated heat assisted magnetic recording (HAMR) transducer, it is known that co-location of the near field optical source and the magnetic write field is desirable. To integrate the PSIM in a heat assisted magnetic recording (HAMR) head, the central part of the grating coupler is absent. This is the region where the magnetic pole is present. Thus, it is desirable that the peak intensity of the incident beam does not lie in the region where the pole is present.